In considering the vegetation of the hills it must be remembered that no plants suitable for forage or large enough for fuel would be left untouched, as the biotic factor is quite as much in evidence in modifying the Biskra flora as that of the other regions visited. As the plants are at present, however, and for whatever definitive causes, there is a considerable variation in numbers and apparently also in kinds. An examination of different parts of the hills by which various exposures as well as other conditions are seen bears out the hypothesis. On the southern slopes, particularly the upper portions, the plant covering is especially poor. The population of an area 16 by 16 meters in the upper south slope included 134 perennials and numerous annuals. Of the species, _Haloxylon scoparium_ was the most numerous; there were also _Dæmia cordata_, _Thymelæa microphylla_, and _Fagonia sinaica_ (figs. 78 and 80). All of these species were small, so that from superficial examination the area appeared fairly barren (fig. 73). When one crosses the hills to the north, so that northern and northwestern exposures are seen, a striking difference is immediately apparent. The northern slopes are much richer in perennials than the opposite facing. The population of an area of the size before taken shows this observation to be correct. On a square 16 by 16 meters, with north exposure, 536 living perennials and numerous annuals were found, with _Haloxylon scoparium_ again the dominant species. The species second in number was _Ferula vesceritensis_, which is to be found chiefly on the north exposure (figs. 74, 75, and 77). There were also a few specimens of _Acanthyllis numidica_, and many annuals, which, unlike those on the south facing, were not in flower. On the lower slopes, in the small and open gulches and the lower portions of the washes and their flood-plains, the plants were most numerous and include a variety of forms, with _Peganum harmala_ the most conspicuous and most numerous.